1. Field of the Invention
The present invention relates to gas sensors which make use of the change in resistance of tin oxide (SnO.sub.2), and more particularly relates to stabilizing the gas sensors resistance over time. The gas sensor according to the present invention is suitable for detection of combustible gases such as methane and butane, and poisonous gases such as carbon monoxide.
2. Brief Description of the Prior Art
Generally gas sensors are made of metallic oxide semiconductors and their resistance value gradually lowers with use. As a result the gas sensors sensitivity to detect gas changes with time.
Japanese patent laid open No. 60-100, 752 discloses the addition of vanadium pentoxide (V.sub.2 O.sub.5) to gas sensors made of SnO.sub.2 to effectively inhibit this change in resistance with time. However, the addition of V.sub.2 O.sub.5 alone does not sufficiently improve the sensors thermal resistance.
The applicant has therefore screened different materials, mainly transition metal oxides, searching for a suitable substitute for V.sub.2 O.sub.5 and has found that only rhenium (Re) satisfies this need.